Functional Connectivity between the Red Nucleus and the Hippocampus Supports the Role of the Hippocampal Formation in Sensorimotor Integration

Experiments were carried out in urethane-anesthetized rats in order to evaluate the hypothesis that the red nucleus has functional connections with the hippocampal formation. Depth profiles of electrical stimulation in Experiment 1 confirmed that stimulation administered to the red nucleus elicited theta field activity in the hippocampal formation with a linear relationship between stimulus intensity and theta frequency. Experiment 2 demonstrated that microinfusion of local anesthetic procaine hydrochloride into the medial septum resulted in a reversible blockade of theta field activity elicited by electrical stimulation of the red nucleus. In Experiment 3, the discharge activity of red nucleus cells was recorded during the field conditions of hippocampal synchrony (theta) and hippocampal asynchrony (LIAlarge amplitude irregular activity). Analysis revealed that 26/46 (56%) of red nucleus cells were theta-related while the remaining 20 (44%) were non-related. The majority of theta-related cells were classified as tonic theta-ON. A brief increase above the basal discharge rate of tonic theta-ON red nucleus cells during LIA predicted the transition from LIA to theta with 400 – 500 ms latency. Furthermore, higher frequency transitional discharges predicted higher theta frequencies, while higher discharge rates during theta predicted shifts to higher theta frequencies. The results supported the conclusion that the red nucleus, traditionally associated with motor functions, is functionally connected with the neural circuitry involved in the generation of theta band oscillation and synchrony in the hippocampal formation, in agreement with the predictions of the sensorimotor integration model of hippocampal function (Bland and Oddie 2001).